package com.zl.mywork.mapper;

import java.util.HashMap;
import java.util.concurrent.ConcurrentHashMap;

public class StudentMapper  {

    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();
        map.put(null,null);
        String s = map.get(null);
        ConcurrentHashMap<String, String> map2 = new ConcurrentHashMap<>();
        map2.put(null,null);
        String s2 = map2.get(null);

    }

}
